Foam resins which contain urethane groups and which are obtained by the reaction of polyisocyanates with particular polyols are widely used in the field of insulations, for the manufacture of structural elements or for upholstery or padding purposes.
It is known to produce cold-curing urethane group-containing foam resins in molds from compounds which contain active hydrogen atoms, preferably polyols, polyisocyanates, water and/or other blowing agents in the presence of emulsifiers, auxiliary agents, catalysts and flame-retarding additives. The function of the emulsifiers and stabilizers is to homogenize the reactants, to facilitate the foaming process and to prevent collapse of the foams after the evolution of gas has ceased. The catalysts ensure that the various processes which take place during formation of the foam are brought into the correct equilibrium with each other and proceed at the proper rate.
Polyethers are frequently used which contain at least two hydroxyl groups and in which at least about 10% of the hydroxyl groups present are primary hydroxyl groups and which generally have molecular weights of from about 750 to about 100,000 and preferably from 4000 to 10,000. These polyethers can be used in combination with certain polyisocyanates to produce cold-curing urethane foams which have the desired physical properties.
The polyisocyanates used may be, for example, the so-called "modified polyisocyanates", e.g. solutions of polyisocyanates which contain biuret groups in polyisocyanates which are free from biuret groups and/or solutions of polyisocyanates containing at least two isocyanate groups and at least one N,N'-disubstituted allophanic acid ester group in polyisocyanates which are free from allophanic acid ester groups and/or solutions of polyisocyanates containing urethane groups in polyisocyanates which are free from urethane groups and/or solutions of polyisocyanates containing isocyanuric acid rings in polyisocyanates which are free from isocyanurate groups.
These cold-curing foam resins with urethane groups have the desired physical properties, but often have defects in the form of bubbles under the surface of the foam. This especially occurs if they are produced by a process which involves simultaneous foaming and shaping, and these defects may also be propagated into the interior of the shaped product. These defects are a serious disadvantage, particularly in the manufacture of shaped parts (which may be used for various purposes, such as the furniture industry or the automobile industry) because the bubbles are clearly seen if the foam is covered with fine textiles. The difficulty cannot be overcome by using commercial polysiloxane-polyalkylene oxide copolymers because they cause irreversible shrinkage even if the stabilizers are only used in minute quantities.
Attempts have been made to overcome the above defects in the production of polyurethane foams by adding certain silicon compounds (see German Offenlegungsschriften Nos. 2,221,811; 2,232,525; 2,337,140 and 2,246,400). These attempts have not so far led to completely satisfactory results. In particular, it has been found that, in many cases, shrinkage of the foam cannot be completely avoided if the silicon compounds specifically described in the said Offenlegungsschriften are used.
German Offenlegungsschriften Nos. 2,211,811 and 2,232,525 (corresponding to British Pat. No. 1,381,571) describe the use of silicon compounds of the formula: ##STR1## wherein a can be 0, 1, 2 and 3.
A process for the production of non-shrinking, cold-curing foam resins which contain urethane groups has now been found which is based on the use of particular silicon compounds, whereby the disadvantages described above may be obviated and faultless foams may be produced.